International observers say it appears part of the MH17 wreckage has been cut into, in what Australian Prime Minister Tony Abbott calls "evidence-tampering on an industrial scale".

"Major pieces, I'm looking at the tailfin ... they do look different than when we first saw them, in that they have been cut into," Organisation for Security and Co-operation spokesman Michael Bociurkiw told the BBC world service.

"One main cone section has been almost split in half."

The OSCE has visited the crash site each day since Friday, though their access to the site was initially very limited.

Two days ago they observed that the cockpit section and part of first class were being cut into with a diesel power saw by uniformed men.

He could not identify who was doing that work.

"The time has come for professionals to be here and do the analysis," Bociurkiw said.

Abbott called it a "cover-up".

"After the crime comes the cover-up," the Australian prime minister was quoted by the Guardian as saying.

"What we have seen is evidence-tampering on an industrial scale, and obviously that has to stop," Abbott said.

He noted not only were "random individuals" seen picking over the site, but heavy equipment had been seen in footage.

The OSCE has facilitated three different groups of experts getting access to the site: first a Ukrainian group, then a Dutch forensic group on Monday, then three Malaysian experts on Tuesday.

Ukraine army spokesman Colonel Andriy Lysenko claimed that Russian military experts, disguised as citizens, had been interfering with evidence at the scene since the crash.

The site is still under separatist control. On Tuesday morning a team of international observers and a crash investigation group including some Malaysian members arrived to continue inspection of the wreckage under the watching eye of armed militiamen.

More than 140 square kilometres have now been examined by Ukraine emergency services since their arrival on Saturday, however professional crash investigators have only recently arrived at the scene.

Meanwhile, Ukraine continues to attack separatist forces in the country's east, even while investigations at the crash site continue, Colonel Lysenko told journalists.

He said the army had committed to a ''non-combat'' area around the crash site with a "radius of 20 kilometres, a diameter of 40 kilometres".